not getting increasing voltage with rpm from the tach amp is not normal. before swapping in another tach, I'd suggest disconnecting the tach from tach amp (so it is not loading the tach amp), and check that you are getting increasing voltage with rpm.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked